1. Field of the Invention
This invention pertains to a travelling wave tube with a helix-type delay line attached to a sleeve through the use of boron nitride dielectric supports.
The invention pertains to the area of travelling wave tubes (TOP's), with a helix-type delay line, i.e., for example, a single helix delay line, of the "ring and bar", "ring and helix" type.
However, to simplify the presentation, the delay line will be assimilated with a single helix in the following.
The helix delay line is placed in a cylindrical sleeve, which is generally made of metal, to which it is attached through the use of dielectric supports.
For travelling wave tubes operating at relatively low power levels, the helix and the supports are assembled in the sleeve by clamping. The helix is made, for example, of tungsten, and the supports are made of quartz, aluminum, beryllium oxide, or boron nitride, for example. The sleeve can be made, for example, of copper of inoxidizable steel.
For travelling wave tubes operating at higher power levels, the helix is soldered to the dielectric supports, which are soldered to the sleeve. In this case, the helix, as well as the sleeve, can be made of copper, and the dielectric supports can be made of beryllium oxide, for example.
Generally, three dielectric supports, regularly distributed at 120 degrees apart, are utilized.
This invention proposes to remedy the problems which occur when boron nitride dielectric supports are utilized.
2. Description of the prior art conductivity and its low dielectric constant, which is about 3 for anisotropic boron nitride; this low dielectric constant prevents energy from concentrating in the dielectric supports and improves the impedance of the coupling.
When boron nitride dielectric supports are utilized for TOP's operating under direct current, a significant fraction of the cathode current is intercepted; this fraction can be as much as 50% of the cathode current. In addition, the fraction of the cathode current which is intercepted can vary in high proportions over time.
When boron nitride dielectric supports are utilized for TOP's operating under impulses, one observes a relatively high helix current, which increases during impulses, and which presents the risk of damaging the helix.
To remedy the problems which have existed for many years in the use of boron nitride dielectric supports, the applicant first coated the dielectric supports with a slightly conductive material, such as graphite. This graphite coating accentuated the problems rather than solving them.